feat(upgrade): provide unit test helpers for wiring up injectors (#16848)

Adds two new helper functions that can be used when unit testing Angular services
that depend upon upgraded AngularJS services, or vice versa.
The functions return a module (AngularJS or NgModule) that is configured to wire up
the Angular and AngularJS injectors without the need to actually bootstrap a full
hybrid application.

This makes it simpler and faster to unit test services.

PR Close #16848

packages/upgrade/BUILD.bazel

@@ -23,6 +23,7 @@ ng_package(
     srcs = [
         "package.json",
         "//packages/upgrade/static:package.json",
+        "//packages/upgrade/static/testing:package.json",
     ],
     entry_point = ":index.ts",
     tags = [
@@ -34,5 +35,6 @@ ng_package(
     deps = [
         ":upgrade",
         "//packages/upgrade/static",
+        "//packages/upgrade/static/testing",
     ],
 )